1. Field of the Invention
The present invention relates to a method for reducing the wave resistance in an airplane by retarding the generation of a shock wave on an upper surface of a main wing of the airplane when the airplane is cruising at a transonic speed.
2. Description of the Related Art
Even if the flying speed of the airplane is equal to or lower than the sonic speed, a shock wave is generated in a range of transonic speed locally exceeding sonic speed by the flow accelerated at a portion of an airframe (see FIG. 20). If the shock wave is generated in this manner on the upper surface of the main wing, the flow speed of the air flow is suddenly decreased from a supersonic speed to a subsonic speed across the shock wave. As a result, a boundary layer is peeled off at a location downstream of the shock wave to generate a following wake which provides a large wave resistance and hence, the main wing is brought into a so-called shock stalling state due to the sudden increase in drag and the sudden decrease in lift force.
The phenomenon of a sudden increase in drag due to the generation of the shock wave is called a drag emanation, and the mach number of a main flow at that time, is called a drag emanating mach number M.sub.DD. When the flying speed of the airplane reaches the drag emanating mach number M.sub.DD, not only is the amount of fuel consumed increased due to the increase in drag, but also the balance of an airframe of the airplane is adversely influenced by the movement of the center of wind pressure. For this reason, it is necessary to retard the generation of the shock wave as much as possible to increase the drag emanating mach number M.sub.DD.
To increase the drag emanating mach number M.sub.DD, the following techniques are conventionally employed:
(1) a wing profile is used which provides a high drag emanating mach number M.sub.DD, PA1 (2) a sweep-back angle is provided in the main wing, and the like.
In an airplane including an engine nacelle for accommodating a gas turbine engine, consideration is taken in the mounting position for the engine nacelle in order to suppress the drag generated due to the aerodynamic interference of the main wing, fuselage and the like with the engine nacelle to the minimum. The reduction of interference drag is generally provided by mounting engine nacelles to laterally opposite sides of a rear portion of the fuselage having a small interference with a main wing in a business jet plane, and by mounting engine nacelles through pylons to a lower surface of the main wing along which air flows at a lower speed as compared with air flow along an upper surface of the main wing in a large-sized passenger airplane.
In general, in the wing profile having a small thickness, the drag emanating mach number M.sub.DD is higher, but if the thickness is decreased, the volume of the main wing is decreased. For this reason, when a fuel tank is provided within the main wing, there is a problem that the amount of fuel carried therein is decreased, and moreover, there is a problem that the structure weight is increased, because a reduction in strength due to the decrease in thickness should be compensated. Therefore, a peaky wing and a supercritical wing have been proposed as a wing profile which inhibits the generation of a shock wave, while ensuring a required thickness by improving the pressure profile on the main wing in the range of transonic speed.
If the sweep-back angle of the main wing is increased, the drag emanating mach number M.sub.DD can be increased, while ensuring the wing thickness to a certain extent, but the following problems are encountered: the stalling characteristic at a lower speed is degraded; the structure weight is increased for opposing a large flexure moment applied to the root of the main wing; and it is difficult to employ a wing of a laminar flow type having a small friction resistance.
The known documents directed to the above problems include U.S. Pat. Nos. 4,311,289; 4,449,680; 4,314,681; 4,171,786 and 3,727,862.
U.S. Pat. No. 4,311,289 describes the prevention of the generation of a shock wave by providing a channel defined by a rear edge of a main wing, a fuselage, an engine nacelle and a pylon. The ratio of the sectional area of a channel outlet to the minimum sectional area of the channel is set at 1:1.065 or less.
U.S. Pat. No. 4,449,680 describes the reduction in effect of interference between an engine nacelle and a main wing by providing a critical contour area (a portion of the engine nacelle) and a non-critical contour area (the other portion of the engine nacelle and a pylon portion) from a critical zone of the main wing and a critical surface region of the engine nacelle, and forming the critical contour area into a shape which is along a streamline in the vicinity of such area and forming the non-critical contour area into a shape which is not along a streamline in the vicinity of such area.
U.S. Pat. No. 4,314,681 proposes the reduction of the generation of a shock wave by disposing a fairing having a characteristic curve from an intermediate portion to a rear edge of a pylon.
U.S. Pat. No. 4,171,786 proposes the avoidance of an increase in drag without use of a pylon by disposing an engine nacelle supported on a fuselage through an auxiliary wing above a main wing, and defining the height position of the engine nacelle with respect to an upper surface of the main wing and the longitudinal position of the engine nacelle with respect to a front edge of the main wing.
U.S. Pat. No. 3,727,862 proposes the prevention of the generation of resonance between an engine and a main wing by supporting the engine nacelle on the upper surface of the main wing with an elastomer interposed therebetween.